1. Field of the Invention
The present invention relates to an optical composite module and a method of assembling the same and, more particularly, to an optical composite module having a pumping module for an optical amplifier doped with a rare earth element and used for optical communication and to a method of assembling the same.
2. Related Background Art
Conventionally, optical transmission using WDM (Wave-length Division Multiplexing) and optical fiber amplifiers require optical parts having optical multiplexing and demultiplexing functions and the like. Various optical modules having these functions have been developed and commercially available. In many cases, multiplexing and demultiplexing functions are realized by dielectric multilayered films formed on prisms or optical plates. The transmission or reflection characteristics of such a dielectric multilayered film are depend on the incident angle of light. For this reason, a prism or an optical plate must be disposed with a high angle precision (almost .+-.0.2.degree. to 1.0.degree.) in an optical module.
As an optical fiber mounted on an optical module, a single mode fiber having a core diameter of about 10 .mu.m is often used. For this reason, in cases of optically coupling optical fibers to each other by an optical module, or optically coupling an optical fiber and a light-emitting element or a light-receiving element, they must be coupled to each other via a lens. A large optical loss occurs, and the predetermined performance of the optical module may not be obtained unless these optical parts, i.e., the optical fiber, the lens, the light-emitting element, the light-receiving element, and the like are mounted in the optical module with high-precision alignment (optical axis adjustment) on the micron or submicron order.
In consideration of long-term reliability after the assembly of the optical module, a highly reliable fixing technique is required to assemble parts with a high precision alignment. In many cases, after an optical fiber, a lens, a light-emitting element, a light-receiving element, and the like are mounted in metal housing, the parts are fixed to a metal housing by YAG laser welding.
FIG. 23 shows the arrangement of a conventional bidirectional pumping optical fiber amplifier. Referring to FIG. 23, an optical fiber amplifier is generally assembled by connecting the fiber pigtails of discrete optical parts such as an optical demultiplexing coupler 100, a WDM coupler 101, an optical isolator 102, an EDF (Erbium Doped Fiber) 103, and a pumping LD (laser diode) module 104 by fusion splicing. Note that input and output monitors 105 and 106 constituted by PDs (photodiodes) are connected to the optical demultiplexing coupler 100.
According to such an optical fiber amplifier, the overall loss of light in the amplifier is large, and a predetermined performance cannot be obtained unless losses in the respective optical parts and the fusion-spliced portions are finely managed. Although various attempts have been made to dispose a prism or an optical plate with a high angle precision, problems have been occurred in terms of mass production and cost because occurred process and assembly techniques are required.
FIG. 24 is a schematic view showing a pumping module located on the rear stage of a conventional optical fiber amplifier, which is disclosed in, e.g., Japanese Patent Laid-Open No. 4-12728. This module corresponds to the broken line portion in FIG. 23. Referring to FIG. 23, an EDF (not shown) doped with a rare earth element such as Er (erbium) is mounted on an optical fiber 6a connected to a portion of an optical separating unit 36 which is located on the left side in FIG. 23. A pumping light source 20 incorporating a semiconductor laser (not shown) is connected to the optical separating unit 36 via an optical fiber 6c. Note that the pumping light source 20 has a terminal 40 for electrical connection to an external unit.
The optical separating unit 36 incorporates a multiplexer 3 for multiplexing transmitted signal light and pumping light from the pumping light source 20, an optical isolator 4 for allowing only light propagating in one direction and blocking light propagating in the opposite direction, and a beam splitter 7 for branching pumping light and causing the resultant light to be incident on a photodiode 8. Note that the optical fibers 6a to 6c are respectively connected to lens holders 44a to 44c via ferrules 41a to 41c. The lens holders 44a to 44c respectively hold lenses 42a to 42c.
When such a conventional pumping module is used, since the pumping light source 20 and the optical separating unit 36 are separately arranged, pumping light from a semiconductor laser is caused to emerge to the optical fiber 6c first and is then collimated to be incident on the optical separating unit 36. Consequently, a large optical loss occurs, and it is not easy to supply the pumping light to the EDF while maintaining a sufficient intensity. In addition, various attempts have been made to realize the functions of a plurality of such optical parts using a single optical module. However, no optical module has reached a satisfactory level in terms of mass production and cost.